What Is a geo-fence
This is a broad explanation of geo-fencing.
A geo-fence (geographical fence) is a pre-determined virtual area in a real word situation. Sounds complicated but it's not. For example if you want to be alerted when your dog wanders out of the garden, take your pet's GPS tracker to the centre of your garden and set a geo-fence of 50 meters for example then as soon as the tracker moves 50 meters from the set point you will be alerted.
Some GPS trackers will allow you to set multiple geo-fences, our personal GPS tracker for example. This is especially useful if the trackers user visits several locations where they need to be supervised i.e. schools, youth clubs, care homes, hospitals, day centres etc.